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From: "Michael A. Terrell"
Subject: Re: Should one worry about reflected current into untuned loads at low
Date: Sun, 08 Dec 2002 00:03:24 -0500
Organization: Have you seen my bench? No, really! Where is it?
X-Mailer: Mozilla 4.61 [en]C-CCK-MCD (Win95; U)
References: <email@example.com> <firstname.lastname@example.org> <email@example.com>
Phil Seakins wrote:
> On Fri, 06 Dec 2002 08:40:18 +0000, Paul Burridge
> >On Thu, 05 Dec 2002 10:49:29 GMT, firstname.lastname@example.org (carltons) opined
> >>This is a kind of tricky question to answer. You can blow up your final
> >>and possibly other stuff by overvoltage or overcurrent due to mismatch,
> >>but if all you're worried about is reflected power then it is simply power
> >>dissipated is DC power in - FORWARD power out. If you exceed your Pd max
> >>for the final, bye bye final. One way of monitoring all of this is by
> >>looking at your final stage current. It should stay in a safe operating
> >>range. Too high and you can damage the final by current and too low, you
> >>can damage the final due to high voltage. Maximum current by mismatch is
> >>very high and almost unlimited, maximum voltage by mismatch can have a two
> >>times supply level going into the final. Look at the specs for the final
> >>and figure this all out as far as a safe operating range. A simple thing
> >>to remember is that reflected power goes into a fairly low VSWR load when
> >>it sees your antenna port. It goes in and turns to heat. Heat is good in
> >>engines, but not very desirable in amplifiers. :-)
> >So basically what you're all saying is that it's 'getawaywithable'
> >provided one choices a final transistor that's sufficiently overrated
> >for the purpose? Okay, so say 5W into a random length of wire with a
> >Vcc of 12V at a signal frequency of 40MHz. Can anyone suggest a
> >suitably overrated device? (within reasonable limits, of course)
> In your original post on this topic you referred to "reflected power
> with RF signals" in a PA system. Further, the topic mentions "untuned
> Clearly, "PA" stands for Public Address. PA systems are only used for
> audio. I am puzzled why you and several of the respondents are
> overlooking this fact. Their is no way you could get any "normal" PA
> system to reproduce frequencies of 40MHz.
> "Tuned loads" and "reflected power" have no relevance in audio
> distribution. These are RF issues.
PA is used for POWER AMPLIFIER as well.
Michael A. Terrell
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